In a pressurized water nuclear power plant, a pressurizer is required to maintain constant pressure in the primary high pressure temperature loop. The pressurizer is generally about half full of water in a liquid state and half full of water vapor in the form of a bubble. As the liquid water is heated by the electric heaters of the pressurizer, the water vapor bubble increases in size, thereby increasing the pressure in the primary system and thus helping to maintain constant plant pressure even when there are variations in the operating conditions of the plant.
The heaters in the pressurizer are located in heater wells that fit into holes in the shell strength wall of the pressurizer. The heater wells are welded to the wall to form a leak proof boundary for the liquid water within the pressurizer.
It is necessary from time to time to evaluate the integrity of the base metal of the heater well in addition to that of the attachment metal weld between the heater well and the shell pressure boundary wall of the pressurized vessel. Ultrasonic signals are used in this evaluation to detect metal cracks and lack of weld fusion, and such an ultrasonic inspection is normally carried out during manufacture of the pressurizer and is repeated in the field after the pressurizer has undergone extensive use to check the integrity of the pressurizer. Conventionally, heat well inspection has been carried out manually, both at the time of manufacture and later in the field after extensive operation. The process is slow and is sometimes non-repeatable when defects are found. Further, this approach is very costly in terms of money as well as in terms of radiation exposure to operating personnel who must hold a manual ultrasonic probe in a nuclear power plant environment.